The present invention relates to a noise filter that is mounted in an electronic device or electronic equipment for removing noise generated in the device or equipment.
Digital technologies are important technologies supporting IT (Information Technology) industries. Recently, digital circuit technologies such as LSI (Large Scale Integration) have been used in not only computers and communication-related devices, but also household electric appliances and vehicle equipment.
However, high-frequency noise currents generated in LSI chips or the like are spread from the LSI chips over wide ranges within circuit boards mounting the LSI chips by electric transmission including inductive coupling with signal wiring or ground wiring on the circuit boards, and further radiated as electromagnetic waves from the signal cables or the like around the circuit boards.
In a circuit comprising an analog circuit portion and a digital circuit portion, electromagnetic interference from the digital circuit portion to the analog circuit portion has become a serious problem.
As a countermeasure therefor, a technique of power supply decoupling is effective wherein an LSI chip as a source of generation of high-frequency current is separated from a DC power supply system in terms of high frequencies. Noise filters such as bypass capacitors have been used hitherto as decoupling elements. The operation principle of the power supply decoupling is simple and clear.
A capacitor conventionally used as a noise filter in an AC circuit forms a two-terminal lumped constant noise filter. A solid electrolytic capacitor, an electric double-layer capacitor, a ceramic capacitor or the like is often used therefor.
When carrying out removal of electrical noise in an AC circuit over a wide frequency band, inasmuch as a frequency band that can be dealt with by one capacitor is relatively narrow, different kinds of capacitors, for example, an aluminum electrolytic capacitor, a tantalum capacitor and a ceramic capacitor having different self-resonance frequencies, are provided in the AC circuit.
Conventionally, however, it has been bothersome to select and design a plurality of noise filters that are used for removing electrical noise of a wide frequency band. In addition, there has been a problem that the use of different kinds of noise filters makes the circuit high in cost, large in size, and heavy in weight.
Further, for dealing with higher-speed and higher-frequency digital circuits, noise filters are desired that can ensure decoupling over a high frequency band and exhibit low impedances even in the high frequency band.
However, the two-terminal lumped constant noise filters have difficulty in maintaining low impedances up to the high frequency band due to self-resonance phenomena of capacitors, and thus are inferior in performance of removing high-frequency band noise.
Therefore, a noise filter is requested that is excellent in noise removing characteristic over a wide band including a high frequency band and that has a small size and a simple structure.
In order to respond to the request mentioned above, attention is given to a transmission line type noise filter, which is connectable between a power supply and an electrical load component such as the LSI chip and can pass coming DC current while attenuating coming AC current.
However, because the DC current to be supplied to the electrical load component passes in the transmission line type noise filter, heat is generated in the transmission line type noise filter. Therefore, the transmission line type noise filter is serious in heat generation for use in an electrical circuit having a large DC current flowing therein, and the life of the transmission line type noise filter is therefore shortened.